Believe it or not, I got my start in B.net gaming-- Diablo and BW to Diablo 2 and WCIII. Great games and some great mobas in the RTS titles, everyone knows and loves em if they've played. But, you know, all this talk for the player base-- the thing was, everyone was held to those 'vacant' standards. That is, unlike League, if I reply to leaving a game by leaving as well, neither party is punished. Later on in WCIII, this usually meant a remake as lobbies were managed by bots. The fact that leaving a game would ruin it and inspire twenty more minutes of waitin dissuaded leavers-- but unlike League, a multitude of games and modes existed. So, usually, you played these games with similar folks. Even competitive games saw a lot of camaraderie: after all, everyone just spent a chunk of time looking for, and waiting for this game to start.
Thing is, in League, if someone leaves they're punished; then, if I leave (because that game is usually ruined) I get punished too. Instead of the game itself being enough to convince people to stay, there's more like a 'Can I Lose' checklist they go through and, with enough points, they simply drop or troll. (This is basically the mentality that keeps me from ranked) The difference here is just that you can't 'remake' like on B.net games. The remake process is guaranteed to take time, and that's after the game finishes or is forfeit. Not only that, should it go sideways the process starts again. And two faulty games back to back isn't out of the question.
The problem isn't a 'crybaby' stance, it's that losing games are now just no fun at all. Most of them are monodirectional stomps over one person getting fed/feeding, meaning that the room for counterplay or turnabouts feels badly. So, while leaving the game freely seems like a good option, it's going to become default answer for a lot of people because there's nothing keeping them there. It's a poor gameplay experience.
Letting people circumvent a bad game is addressing the wrong issues. Unfortunately, if we go that step in that direction, most likely is that we'll just have more games ruined-- tons more people will just leave games they don't like, jumping ship after their lane first bloods. People are so unwilling to play a losing game in any capacity. Worse yet, these losses are in part caused by players; so if they're losses before, they'll be so after letting these players just leave. Now we cross into clubs leaving games as a whole, dynaQ parties group quitting. It becomes another tool; the damage is done and the community throws a fit at any game that doesn't end in their victory.
Yes, you're right-- B.net had a far different and arguably more successful system. But introducing that structure here, even in part, is giving the wrong tools to the wrong people. This might work in higher ranks where the whole game is on one page. But norms and lower queues and even some of the higher ones, you'll just be giving trolls more ability to force games into an early or unwarranted end. It's not like B.net, where anyone in BGH is in to play BGH-- you might have people who quit after a rush or employ gimmicky strats but in the end, League has a totally different threshold for tolerance. They're all playing League, but maybe this guy here doesn't play games where Zed or Yasuo are picked. Your JG doesn't play against Jax or Xin. Your top laner refuses to play with stealth champs. The enemy team scored first blood, the enemy got a penta-- there are a thousand reasons players throw games in many ways, and I just think outright letting them do it will stop dissuading the ones who would do so if they didn't get punished. Losing is less and less a thing that happens because your enemies fought for and earned a victory. In a different League, your plan holds--- but by both our admissions, this is a great-power-great-responsibility scenario in which the intent to fix a problem would exacerbate it... B.net had a different tone and ultimately was a different beast, from BGH to Golems to Zone Control, it had myriad options and types of gamers. League is just one game, the moba, always.
As for what exactly toxicity is in League, that's a whole other discussion and honestly I think it dips very heavily into sociological trends and nobody really wants to open that can of worms in the scope of something like League. My short version is just that it's a gap in understanding that loses sight of why toxicity is such a problem... And most importantly to whom.